Osteosynthesis comprises all methods which make it possible to hold two bone structures in place following a fracture, an arthrodesis or an osteotomy. It is used, for example, when the reduction (that is to say realignment of the bone ends) cannot be done by external manipulation or when the two fragments are not stable. It uses various instruments for holding together the two fragments of the bone (or a bone and an implant), such as plates, rods, nails, screws, pins, staples, etc. The aim is to achieve consolidation of the bone in the anatomical position.
Osteosynthesis with the aid of an instrument such as a plate screwed by means of one or more screws has been used in bone surgery for decades. Holding of the bone is then ensured by the plate, which is fixed to the bone fragments to be joined with the aid of a plurality of screws passing through said instrument and the bone tissue. The screws used for holding the instrument may be of different diameters and different lengths, depending on the shape of the instrument and the position of the implantation.
These screws conventionally comprise a relief in the upper part of the head, which will cooperate with a screwdriver whose tip is of complementary shape: screw with a slotted or cross head, or a hollow head (for example cooperating with a male screwdriver with 6 contact surfaces). They are made available to the surgeon at the time of the intervention at the same time as specific equipment dedicated to positioning the fixation system, generally comprising drills, a screw tap and a screwdriver.
Osteosynthesis using an instrument such as a plate associated with one or more screws is regarded as a reliable means for obtaining good bone consolidation, when it can be carried out properly. To this end, a certain number of conditions need to be satisfied, or at least sought, among which the quality of the screwing has a very important role. It is essential for the instrument to be tightly secured to the bone as soon as it is implanted and over the course of time. It is therefore imperative for the screws to remain in place despite the vibrations and other stresses which may create a play and lead to their loosening. The screws therefore need to be securely anchored both to the bone and to the instrument.
It is also necessary to be able to orientate the screw at a suitable angle, which the surgeon will select according to the bone parameters of the patient. The screws may thus be inserted along an axis perpendicular to the instrument, but it is often necessary to impart a certain inclination to them with respect to a right angle, particularly in order to reach a bone fragment far from the fracture region. For this reason, the head of the screw cannot be fully engaged in the cavity (recess) provided for this purpose in the instrument and projects on the surface.
The devices formed by an osteosynthesis instrument such as a plate, and screws, provided to surgeons have for a long time been made of metal, generally titanium or stainless steel. Currently, the best metal devices are locked and orientable devices. Each cavity intended to receive a screw head then consists of a lug having a certain degree of mobility relative to the instrument. The locking is carried out using a second screw thread formed on the lower part of the head of the screw, the first screw thread corresponding to the shaft of the screw engaging with the bone. This second screw thread is received in a complementary screw thread formed at the lug. When the lug is mounted in an orientable fashion, this locking device may furthermore be inclined in a suitable direction, commonly in a radius of the order of 0 to 15° in all directions.
The metals or metal alloys used, although tolerated well by the body, have two drawbacks. On the one hand, since the instrument, often in the form of a plate, is only rarely removed (in about 15% of cases), it must participate in the long term in the kinematic function of the organ which it has made it possible to resolve. However, its presence has the effect that the mechanical stresses are not transmitted homogeneously into the bone in question. This results in weakening of the bone at the points of contact between the bone and the instrument, namely essentially at the screws. Secondary fractures are therefore frequent.
In order to overcome this problem, osteosynthesis devices comprising an instrument, for example in the form of a plate, and one or more screws made of polymers have been proposed. Biocompatible polymers such as PEEK (polyether ether ketone), implantable polyamides, UHMWPE (ultrahigh molecular weight polyethylene) or PETs (polyethylene terephthalates), may be used. They offer the advantage of less rigidity, which reduces the mechanical stresses imposed on the bone. More recently, an alternative has been proposed which is based on the use of resorbable polymers, which have the advantage of progressively disappearing in the body, but at the end of a time delay sufficient for the bone to have recovered its solidity. For example, polymers belonging to the family of PLAs (polylactic acids), PCLs (polycaprolactones), PDSs (polydioxanones) and PGAs (polyglycolic acids), as well as copolymers thereof, are known.
Whether or not they are resorbable, all these polymers have a major drawback: they are radiotransparent. For this reason, it is not possible to monitor their position during their implantation or subsequently with radiographic techniques. A new generation of osteosynthesis devices has recently been developed by the Applicant, which are produced with the aid of a composite mixture of materials that progressively degrade, comprising a polymer or copolymer compound and an inorganic filler provided by a ceramic.
There are therefore now a range of nonmetallic osteosynthesis implants offering new advantages to the surgeon. However, a certain number of drawbacks remain to be overcome.
It has been seen that, in metal osteosynthesis devices, the connection between the instrument and the or each screw is reinforced by locking. However, devices produced on the basis of polymers (including a composite mixture of polymer and ceramic) cannot use this type of fixation. This is because the techniques for shaping polymer materials do not make it possible to produce a thin screw thread on the surface, which in any event would not withstand the mechanical stresses (essentially friction) during screwing. The polymer screws known to date comprise a conical or hemispherical head and are placed perpendicular to the overall plane of the instrument, often a plate, or with a maximum angle of 5° with respect to this plane. It is not possible to lock them like metal screws. This constitutes a significant drawback of polymers compared with metal.
In order to solve this problem, a system for holding polymer-based osteosynthesis instruments has been developed, using headless screws, also polymer-based, these being fitted with the aid of a screwdriver designed and dedicated specifically to these screws.
It has been found that it is possible to carry out locking of the screws on synthetic polymer materials by welding the surfaces in contact. Specifically, the nature of the materials used can be exploited in order, on the one hand, to cut the screw after insertion to a desired depth in the bone in order to remove the protruding rear part after insertion, and on the other hand to weld the screw and the associated instrument together by surface fusion of the materials, induced by an electric knife. The shaft of the screw is thus melted in the surface in the lug of the instrument through which it passes.
This being the case, the rear part of the screw is eliminated, making sure that the screw does not protrude from the surface of the bone. The screw can then be inserted into the instrument in any direction with a high inclination (which may be up to 30°), without creating a perturbing projection, while an osteosynthesis device with metal screws allows maximum inclinations of only 15°. Thus, unexpectedly, a lockable and orientable device is obtained which performs better than the metal screw/instrument device.
However, such a device raises a new problem, which is the basis of the present invention. It is in fact indispensable to monitor the insertion depth of the screws into the bone tissue. This is particularly important because, in numerous indications, it is recommended for the screws to pass fully through the bone in order to grip the external walls of the cortical bone at two opposite points. The cortical bone refers to the external wall of the bones, which imparts rigidity and elasticity to them. It is formed of a dense layer of calcified tissue, which surrounds the medullary cavity filled with bone marrow. One difficulty for the surgeon then resides in fine assessment of the insertion length which is sufficient for the bicortical fixation, but without the screw, referred to as a bicortical screw, protruding excessively from the surface of the bone.
Conventionally, during an operation, the surgeon will determine the necessary screw length, in particular by means of a depth gauge, then select screws of suitable length from a range of screws available to him, and fit them by screwing thoroughly until the screwing end of the screw, most often its head, is blocked in contact with the plate. Such a method of fixing the screws involves the use of screws provided with a head, while such a procedure cannot be adopted in the case of a screw intended to be cut.